Hotrodders and Streetrodders put new filler necks/gas doors in all the time. Depending on how good he wants it to look, he can salvage a gas door from a salvage yard or buy it aftermarket. Just cut a piece of cardboard with the contour of the sheetmetal where he wants the door and go looking. Take along a battery powered saws-all to cut the piece out with metal to spare. As to get from the gas door to the tank, depending on length, a short rubber hose of the right size (measure the neck on the tank). Then a piece of pipe bent at home or a muffler shop, if you can't find it also at the yard. Another short piece of hose from pipe to gas door neck and you got it. If he just want a filler hole with a gas cap it get even easier. Remember to put a vent in. We weld a small metal tube (1/4") to the top of the metal pipe close to the gas hole. Then slide a rubber hose over that, then mount the other end higher somewhere and put a cheap gas filter on that end to keep water out. Good luck.
